The present invention relates to coded modulation techniques to a
Generally, the application of trellis-coded modulation techniques to a communications system results in improvements in both the communications system's power and bandwidth efficiencies. Trellis-coded modulation techniques have been applied to communications systems that are characterized by additive white Gaussian noise (AWGN) and/or a "fading" channel. An example of an AWGN channel is the transmission path between commercial telephone-line modems. An example of a "fading" channel is that of cellular mobile radio where the received signal can sometimes be too weak to carry any useful information.
Trellis-coded modulation is often used in conjunction with a 2N-dimensional constellation, where N.gtoreq.1 and each 2N-dimensional symbol from the constellation represents a number of information bits. For cases of N&gt;1, in particular, a 2N-dimensional constellation can be formed by concatenating N/M constituent 2M-dimensional constellations. Usually, M=1 so that N constituent two-dimensional (2D) constellations are used.
However, in a particular communications system, the selected 2N-dimentsional symbol and bit rates may result in a fractional number of bits per 2N-dimensional symbol, i.e., a fractional bit rate. For example, in telephone voiceband applications a 2D symbol rate of 2,742.86 (19,200/7) is typical. At the same time, however, most present-day data transmission is carried out at a selected one of a limited set of standard bit rates, e.g., 14,400 bits/sec. Use of a 2D constellation with the above symbol and bit rates results in a fractional bit rate of 14,400/2,742.86=5.25 bits per 2D symbol.
In applying a trellis-coded modulation technique to a fractional bit rate communications system, the prior art has taught that the fractional information bits cannot be trellis-encoded, see, U.S. Pat. No. 4,941,154, issued Jul. 10, 1990 to L.-F. Wei. As shown in the above-mentioned prior art, for an average fractional rate of m information bits per 2N-dimensional symbol interval, where m=n+k, and n&gt;0 and is an integer, and k&gt;0 and is not an integer, a trellis encoder is used to encode the n, out of the n+k, information bits in each 2N-dimensional symbol interval into n+1 encoded bits. The later select one out of 2.sup.n+1 subsets, each subset comprising 2N-dimensional symbols from a 2N-dimensional constellation. However, the remaining fractional information bits, k, are not trellis-encoded but are precoded to become an integral number k' of bits, which are then used to select a 2N-dimensional symbol from the selected subset for transmission.